1. Field of the Invention
The present invention relates to a composite retarder plate and an optically compensatory polarizing plate which can compensate highly for birefringence due to liquid crystal so that a liquid-crystal display device excellent in view field angle and contrast can be formed.
The present application is based on Japanese Patent Application No. Hei. 11-221395, which is incorporated herein by reference.
2. Description of the Related Art
Enlargement of the view field angle and increase of contrast have been required with the diffusion of liquid-crystal display devices (LCD) to television sets, personal computer monitors, etc. In the meanwhile, there was a proposal of making a retarder plate compensate for a phase difference due to birefringence of liquid crystal to improve visibility characteristic, for example, to achieve enlargement of the view field angle of good visibility in a TN-LCD and to achieve monochrome display owing to coloring compensation in an STN-LCD. The background-art compensatory plate was, however, insufficient to cope with the phase difference characteristic of liquid crystal. There was a problem that the improvement of the visibility characteristic of liquid crystal by the background-art compensatory plate was not satisfactory.
Incidentally, in the TN-LCD, a Wide View film (tradename, made by Fuji Photo Film Co., Ltd.) and an NH film (tradename, made by Nippon Oil Co., Ltd.) are known as compensatory plates for enlarging the view field angle. There was, however, a problem in tone reversal, remarkable lowering of contrast at a view field angle of 60 degrees or higher, coloring in a monochrome level, and so on.
On the other hand, in the STN-LCD, a superposition type compensatory plate made of a laminate of retarder plates of polymers exhibiting positive birefringence characteristic is known (Unexamined Japanese Patent Publication No. Hei. 7-306406). There was, however, a problem that coloring compensation was so insufficient that coloring occurred in monochrome display. Moreover, in a TFT-LCD, enlargement of the view field angle is required even in a vertical arrangement (VA) mode, a horizontal arrangement mode, etc. as well as a TN mode. The aforementioned compensatory plate was, however, hard to be adapted to the enlargement of the view field angle.
An object of the present invention is to develop a retarder plate which can compensate highly for a phase difference due to birefringence of liquid crystal so that a liquid-crystal display device excellent in view field angle, in contrast and in uniformity thereof can be formed.
According to the present invention, there is provided a composite retarder plate comprising a combination of at least one retarder film and at least one liquid-crystal phase sheet, the liquid-crystal phase sheet including a transparent substrate, and a liquid-crystal polymer layer provided on the transparent substrate, wherein: at least one of refractive indices nx, ny and nz of the retarder film is different from the other refractive indices when nx, ny and nz are main refractive indices (nxxe2x89xa7ny) in in-plane directions and a refractive index in a direction of the thickness of the composite retarder plate, respectively; all the refractive indices nx, ny and nz of the transparent substrate are different from one another; the retarder film, the transparent substrate and the liquid-crystal polymer layer are different in wavelength dependence of birefringence; the retarder film and the transparent substrate are different in Nz defined by the expression (nxxe2x88x92nz)/(nxxe2x88x92ny); and each of the retarder film and the transparent substrate is constituted by a film having non-liquid-crystal high molecules oriented.
According to the present invention, there is provided an optically compensatory polarizing plate, wherein the aforementioned composite retarder plate is provided on a single side of an absorption type polarizing plate. There is further provided a liquid-crystal display device, wherein the aforementioned optically compensatory polarizing plate is disposed on at least one side of the liquid-crystal cell.
According to the present invention, a composite of phase difference layers is made on the basis of combination of a retarder film, a transparent substrate and a liquid-crystal polymer layer different in wavelength dependence (wavelength dispersion) of birefringence and on the basis of combination of the retarder film and the transparent substrate different in part or all of the refractive indices in in-plane and thickness directions and different in Nz. Hence, wavelength dependence is controlled by the synthesis of the phase difference layers so that a retarder plate capable of compensating highly for a phase difference due to birefringence of liquid crystal can be obtained. Particularly, a liquid-crystal display device which can compensate highly for coloring due to the view field angle and which is excellent in view field angle, contrast and uniformity thereof can be formed.
That is, birefringence of liquid crystal varies in accordance with the state of orientation even in the case of one and the same liquid crystal. For the purpose of compensating for such birefringence and particularly for the purpose of compensating for coloring owing to the view field angle, it is necessary to cope with wavelength dependence on a phase difference and coloring as well as to cope with the phase difference and the change of the viewing angle. A retarder plate having enriched phase difference characteristic can be obtained by combination of materials different in wavelength dispersion and particularly by combination of materials different in Nz with respect to wavelength dispersion of xcex94nxy and xcex94nxz which are defined by (nxxe2x88x92ny) and (nxxe2x88x92nz) respectively and which have large influence on the change of the viewing angle. Hence, it is possible to cope highly with the phase difference due to birefringence of liquid crystal, the change due to the viewing angle and the wavelength dependence of these characteristics. Hence, accuracy in compensation for coloring due to the view field angle can be improved greatly. Accordingly, it is supposed that shortage of compensation by the background-art compensatory plate is based on the fact that the background-art compensatory plate cannot be adapted sufficiently to wavelength dependence of the phase difference and the change of the viewing angle.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.